Hi Lark.
You can create an entry (input) in Decision to receive a word that will be sent by Workflow.
In Decision you can check the word (password).
About the send word (password) you can use a private key to send a word and a public key to view a word (password).
If want something more simple you can use a public key in your workflow or just create a input in your decision and check de word (password).
I sugest you use de key to improve your security comunication between Workflow and Decision.
Note. LogicaBase.Components.Encryption.dll is very useful
Regards.